The invention relates generally to large area solid state imager devices and specifically to imager devices having reduced photodiode sidewall leakage.
Solid state imager devices are used for radiation imaging (e.g., x-rays), such as for use in medical diagnostics. Such solid state imager devices commonly comprise arrays of photosensor elements with associated switching elements arranged in rows and columns, with the photosensor elements being addressed by rows of scan lines (also referred to as address lines) and columns of data lines (also referred to as read-out lines). Typically, the photosensor elements are photodiodes, and the switching elements are thin film field effect transistors (FETs or TFTs).
One of several factors that can influence solid state imager performance is the amount or level of photodiode reverse bias leakage. In the solid state imager devices to which the present invention is directed, the photodiode leakage is regarded as having two components, sidewall leakage (from surfaces of the photodiode with substantial incline to the substrate) and area leakage (from photodiode surfaces substantially parallel to the substrate).
In imager devices to be used in medical applications, the photodiodes in a representative pixel typically have approximately square dimensions on the order of about 0.1 to 0.4 mm in length and width. At such sizes, the sidewall leakage component is of the same order of magnitude as the area leakage component; thus both components contribute significantly to the leakage, thereby degrading the performance of the imager. For example, in radiographic applications, the exposure time (between read outs of the array) can be as long as two (2) seconds, and a photodiode leakage of even 1 pA will degrade performance significantly by saturation of the charge that the photodiode is able to collect, and by contributing to imager noise and offset drift associated with the leakage current.
The potential severity of the photodiode leakage problem is best appreciated when it is considered that the photodiodes may operate under about 10V common bias, at a temperature somewhat above room temperature due to heat generated by surrounding electronic components. Further, the imager device may have on the order of one million (1xc3x97106) photodiodes, of which a high percentage must not be leaky in order to avoid having an excessive number of bad pixels. It is therefore desirable to reduce the reverse bias leakage of the imager photodiodes to the extent practicable, while avoiding an increase in the cost and/or complexity of the imager fabrication process, and while avoiding any degradation in performance or reliability.
A solid state imager is provided that comprises an array of gated photodiodes. The imager comprises a plurality of photosensor pixels arranged in a pixel array, and each of the photosensor pixels includes a photodiode having a side wall, the sidewall having a gate dielectric layer disposed thereon, and a field plate disposed around the photodiode body. The filed plate comprises amorphous silicon disposed on the gate dielectric layer and extends substantially completely around the sidewall of said photodiode. A method of fabricating the gated photodiode array is also provided.